The structural, vibrational, and electronic properties of silica-supported Ziegler-Natta catalysts industrially relevant for polyethylene production were investigated in detail by means of a multitechnical approach at each step of catalyst preparation, including precatalyst activation. In the precatalyst, the TiClx phase is grafted mainly to the silica surface and almost independent of the supported MgClx phase. However, the subsequent activation by means of an aluminum-alkyl compound causes important changes to both the supported MgClx phase and the TiClx phase. The resulting catalyst is entirely reconstructed so that most of the titanium sites are detached from the silica surface and in interaction with a highly dispersed MgCl2 phase, thus mimicking the most famous and highly investigated Ziegler-Natta catalysts (not silica-supported). For the first time, the catalyst performances were monitored by means of in situ FT-IR spectroscopy in transmission mode, simulating industrially significant polymerization conditions (i.e., ethylene is fed onto the catalyst in the presence of the activator and the solvent). These results demonstrate that it is now possible to achieve a complete description at a molecular level of all the constituents of Ziegler-Natta catalysts and at each step of the catalyst preparation.